1. Technical Field. This invention relates generally to catheters adapted to be introduced into a living body, and more particularly to a catheter featuring a new and improved soft tip construction.
2. Background Information. Soft tip catheters, such as coronary angiography and guiding catheters, often employ a soft tip as a precautionary measure. Intended for less invasive vascular procedures, these catheters include the soft tip to avoid injury to vulnerable vessels and arteries. In addition, the soft tip reduces trauma in certain guiding catheter procedures that lodge the catheter tip in the coronary ostium. Thus, the soft tip aspect of catheter design is of recognized significance and each detail of soft tip construction of corresponding importance.
Apart from the soft tip, however, the catheter must exhibit other important attributes. One of these is commonly referred to as torque control, i.e., the ability to transmit a twisting force along its length. Sufficient torque control enables carefully controlled maneuvering of the catheter by the application of twisting forces at the proximal end of the catheter that are transmitted along the catheter axis towards the distal end. However, the features of existing catheter designs that provide greater torque control often complicate addition of a soft tip.
For example, U.S. Pat. No. 4,385,635 to Ruiz describes a catheter composed of a urethane jacketed polyamide tube that provides a degree of reinforcement to the catheter tube for better torque control. However, the soft tip is provided by forming it from the outer layer, and the manner in which the inner layer tapers to zero at a point set back from the distal end results in a discontinuity in the coefficient of friction along the tube interior. This interferes with advancement of another device, such as a probe or inner catheter, within the catheter tube.
U.S. Pat. No. 4,238,447 to Flynn describes another integrally formed soft tip construction employing a multilayered catheter tube. Unlike the Ruiz design, the inner surface of the catheter tube extends fully to the distal end to avoid any abrupt change in coefficient of friction. However, the cross section of the inner and outer layers vary over the length of the catheter tube, and it otherwise suffers from the inconvenience and expense of fabrication common to integral designs. This common concern results, in part, from the need to form the tip with the catheter tube. Thus, it is not possible to cut a selected length from preformed tubing stock and then supply the desired soft tip.
U.S. Pat. No. 4,563,181 to Wijayarathna overcomes some of these problems with a soft catheter tip butt-fused in end-to-end relationship to a nylon catheter tube body, non-reinforced monotubular structures having a difference in Shore A hardness greater than 10 being used in both the tip and body portions. Although providing a soft tip without integral construction, this arrangement results in an abrupt interface between tip and body that often incurs stresses in use that buckle the catheter at the interface or cause the tip to break off. Thus, the tip may break off during deep engagement with severe consequences to the patient.
Despite the use of a lap joint for attaching the soft tip described in U.S. Pat. No. 4,531,943 to Van Tassel, this design is also vulnerable to tip breakage along the portion of the joint disposed generally perpendicular to the catheter tube axis. As in the Wijayarathna et al. construction, external forces applied against the tip perpendicular to the catheter tube axis tend to pull the tip apart from the body along the perpendicularly disposed portion of the joint. In addition, the abrupt changes in cross section result in stress concentrations. When these forces overcome the bond, the tip breaks off.
The perpendicularly disposed joint also frustrates use of a multilayered catheter tube body for better torque control. It does so because the layers of a multilayered catheter tube present, in cross section, small, dissimilar areas to which to bond the tip, and this compounds the difficulty of providing a strong bond to each layer that has sufficient surface area to withstand the external forces applied. Thus, existing soft tip designs using a separate tip often employ catheter tubes to which the tip can be bonded advantageously, but these often exhibit softness in the warmth of the body interior that actually reduces torque control.
Therefore, it is desirable to have a new and improved soft tip catheter that overcomes these concerns--one employing a catheter tube exhibiting better torque control that includes a separate soft tip attached to the catheter tube with a better joint in a manner enabling more convenient and less expensive fabrication.